1. Field of the Invention
The present invention relates to an ionization chamber. More particularly, it relates to an ionization chamber which prevents slight deviations of parts caused by manufacturing error and differences due to thermal expansions of the parts for the ionization chamber.
2. Description of the Prior Art
It has been proposed to use an ionization chamber as shown in FIG. 1.
In FIG. 1, a cylindrical outer electrode (2) is coaxially arranged with a cylindrical inner electrode (1) and the outer electrode (2) is covered by an outer casing (3). A disc bottom plate or first edge plate, (4) is mounted at one end of the outer casing (3) by welding etc. to close the opening of the outer casing and a disc top plate (5) is mounted at the upper end of the outer casing (3) by welding etc. to close the opening of the outer casing. A holding plate or second edge plate (6), which is a disc concentric with the disc top plate (5), is fitted into the inside edge of the outer casing (3) adjacent to the top plate (5) and a shoulder (3a) of part (3) of the outer casing. The inner electrode (1) is electrically insulated from the holding plate (6) by ring insulators (7),(8) and it is held in the fitting structure.
On the other hand, the outer electrode (2) is electrically insulated from the bottom plate (4) by ring insulators or first and second cylindrical guides, (9), (10), respectively and it is held in the fitting structure. Cylindrical insulators (11), (12) are mounted on the holding plate (6) by means of threads. Output terminal parts (13), (14), each including a cylindrical insulator, are mounted on the top plate (5).
The inner electrode (1) and the outer electrode (2) are electrically connected to an output terminal part (13) and an output terminal part (14) through lead wires (15), (16), respectively. The lead wires (15), (16) pass through the insulators (11), (12) and the holding plate (6). Springs (17), (18) are respectively fitted between the insulators (9), (10) and the bottom plate (4). The outer casing (3), the bottom plate (4), the top plate (5), and the output terminal parts (13), (14) are bonded in an air-tight condition with each other to form an air-tight chamber. An ionizable gas fills the chamber within the sealing structure and can move among all spaces in the chamber. The bottom plate (4), the top plate (5) and the holding plate (6) are side plates for closing the openings of the outer casing (3).
The springs (17), (18) are used not only for absorbing errors of the size of the parts of the ionization chamber, but also for absorbing differences due to thermal expansions of the parts in the axial direction at high temperature.
In the preparation of the ionization chamber, the ionization chamber is assembled and heated in a vacuum to a temperature higher than its normal operating temperature for degasing the parts before filling the chamber with an ionizable gas. The springs (17), (18) should also absorb differences due to thermal expansions in the axial direction which is caused in the abovementioned step.
When the springs are heated to a high temperature, a relaxation phenomenon occurs which causes the free length of the springs to become shorter. As the result, the pushing force is decreased. Sometimes, the relaxation at high temperatures is remarkably large. When the device is cooled to a low temperature, a slight looseness of the parts may occur. For example, when the ionization chamber is heated to a high temperature and is then cooled, the ionization chamber is cooled from the outer surface to the higher temperature inner parts of the chamber resulting in large amounts of thermal expansion because it takes a certain amount of time for the thermal conduction to the inside of the ionization chamber. Therefore, the spring (17) used inside is compressed by stronger pressure than that of the temperature equilibrium condition to cause a large relaxation.
In the ionization chamber having a spring, the relaxation of the spring is caused by use at high temperatures or by thermal cycles. The pushing force of the spring is decreased. Sometimes, a looseness is caused, whereby the relative position of the two electrodes is changed by a small outer force to affect the characteristics of the ionization box. Moreover, microphonic noise is caused by shifting the electrodes, thereby disturbing the measurement of radioactive rays by the ionization chamber. These disadvantages are found in the conventional devices.
In the conventional ionization chamber, the output terminal parts (13), (14) are made by metal-ceramic sealing means.
The metal-ceramic sealing means have been widely used as a through type terminal for electric signals in order to transmit electric signals such as current and voltage or electric power through a wall of a closed casing of a vacuum device or an ionization chamber. Various kinds of structures of the sealing parts have been proposed. It is recognized that a cylindrical metal-ceramic sealing means, such as the cylindrical ceramic peripheral part bonded inside a cylindrical sealing metal shown in FIG. 2, is optimum.
In FIG. 2, a cylindrical ceramic part (131) and a cylindrical sealing metal part (132), are bonded at a bonding part (133) between the ceramic part (131) and the sealing metal part (132). The ceramic part is usually bonded to the sealing metal part by forming a metallizing layer on the surface of the ceramic part and plating it with nickel and brazing it with the sealing metal part with braze suitable for the brazing temperature.
Suitable ceramics used for this purpose include forsterite and alumina.
Suitable sealing metals are metals having thermal coefficients substantially similar to that of ceramic and include nickel, iron, cobalt alloy and iron-nickel alloy.
The metal-ceramic sealing terminals and the sealing means which can be satisfactorily used at a temperature ranging from room temperature to about 400.degree. C. have been commercially available.
However, it has been required to use a metal-ceramic sealing terminal which is durable at 700.degree. C. to 750.degree. C., in a high temperature ionization chamber or a high temperature vacuum casing. It has been difficult to maintain the seal with safety at such high temperature by the conventional technology.
A pressurized gas at higher than 3 atm. is used as an ionizable gas in many cases. At high temperature, the metal-ceramic sealing terminal is sometimes broken thereby causing faults. Efforts have been made for improving the strength of the metal-ceramic sealing terminal at high temperature. However, it has been difficult to conform the thermal expansion of the metal (131) with that of the ceramic (132) in the metal-ceramic sealing terminal at wide range of temperature. When the thickness of the sealing metal is increased to increase the high temperature durability and to increase the strength of the sealing metal, the stress at the bonding part caused by the difference of thermal expansions of the ceramic is increased so as to peel off the bonding part.
The sealing metal special alloy has a Curie point, and the thermal expansion coefficient at temperatures higher than the Curie point is highly different from temperatures lower than the Curie point. On the other hand, the thermal expansion coefficient of the ceramic gradually varies depending upon the temperature. It is impossible to conform the thermal expansions of the sealing metal and the ceramic over wide temperature ranges. A peeling-off of the bonding part and the crackings of the sealing metal are caused by applying thermal cycles thereby causing leakage.